In navigation tasks, the addresser often gives ambiguous route instructions.  This paper describes the factor of ambiguity in the route instructions, and discusses the way of developing an interactive dialogue system.  We introduce action vectors, which are points where the addressee took some action according to the given instructions. In our proposal, ambiguity in interpreting route instructions is explained by indeterminacy in the selection of an action vector.  Action vectors enable us to provide a more reliable dialogue system that can interact with users more naturally.